1. Field of the Invention
The present invention relates to a method for cleaning the catheter of an implanted medication infusion system, the system being of the type having a medication container and a pump for pumping medication from the medication container into the patient via a catheter connected to the output of the pump, with a flushing port arranged downstream of the pump and upstream of the catheter for flushing a cleaning fluid through the flushing port and out into the catheter.
2. Description of the Prior Art
Current implantable medication infusion systems include a dosing unit; comprising a pump and a medication container, attendant electronics, a source of energy and a catheter which delivers medication to the intended site in the patient. The tip of the catheter can be located e.g., in a blood vessel, inside an organ, in body tissue or in the peritoneum. The internal diameter of the catheter is small, typically 0.3 mm, for a number of reasons, and occlusion of the catheter is the most common cause of faults in the system. Experience has shown that it takes about 12 months for occlusion or overgrowth of insulin catheters to develop in known systems. This time can vary widely from one person to another but is unacceptably brief in view of the approximate 5-year service life of the infusion system. Therefore either the catheter must be replaced a number of times during the system's service life, a measure requiring surgery, or attempts must be made to flush out the blockage with a neutral solution harmless to the patient.
Remedying catheter blockage by replacing the catheter requires surgery, as noted above, and is relatively expensive and infection in the implant pocket is also a risk, removal of the entire implant then often being necessary, causing the patient discomfort and inconvenience and requiring a special appointment to remove stitches.
Flushing away blockage in the catheter can sometimes be successful. The infusion system must then be equipped with a flushing port with a rubber septum through which a transcutaneous cannula is inserted, so a syringe can be used for flushing with a suitable solution, harmless to the patient, at a high pressure of about 10-20 bars. In this way the catheter occlusion can be forced out and possible overgrowth on the tip of the catheter can be pushed away. Although less invasive than surgery, this method still presents risks to the patient and to the implanted system.
This is in part because modified medication, such as modified insulin, crystallized insulin etc. which is often a major component in the occlusion, could be flushed into the patient's body with the risk that antibodies are formed in the patient against the medication as a negative consequence. There is also a considerable risk of damage to or destruction of the catheter, the catheter connection and the flushing port septum, or the catheter tip could be, in effect, "blown off" by the high pressure. Damage to parts on the output side of the pump by the high pressure is another risk. Aging of the polymer materials exposed to body fluids increases this risk with the age of the catheter.
Moreover, at every flushing occasion the patient receives a relatively high dose of medication, corresponding to the total volume of the flushing port and the catheter. This dose also depends on the concentration of the medication. In the case of insulin, it may be necessary to balance this dose with a concomitant glucose infusion. The aforementioned total volume can typically amount to 100 .mu.l, the momentary dose then being 10 U of insulin when a solution of 100 U/ml of insulin is used, and 40 U of insulin when 400 U/ml insulin is used. Such doses could be directly fatal to many patients if the simultaneous glucose supply is insufficient.
Further, after the flushing event the flushing port and the catheter are filled with, typically, 100 .mu.l of neutral liquid. When the pump is reactivated, the medication therefore mixes with this liquid in the flushing port, and the concentration of the pumped medication will rise only gradually to the correct value. This variation in the concentration is difficult to predict, since there is no active stirring, and the concentration of the pumped medication will fluctuate for a longer period of time with deviations in the prescribed insulin flow as a result.
Flushing with pressurized liquid at high pressure can at best remove a mechanical blockage in the catheter and restore the flow through the catheter. Nonetheless, deposits and residues of precipitated medication and adsorbed proteins will almost certainly remain on the interior surfaces of the catheter and these residues will then serve as "seeds" for new deposits of medication and will cause new catheter blockages in a relatively short time.